Apparatus for measuring liquid level or specific gravity



Feb. 1, 1949. w. H. HOWE 5 APPARATUS FOR MEASURING LIQUID LEVEL OR SPECIFIC GRAVITY Filed Feb. 26, 1945 3 Sheets-Sheet l c a ZNVENTOR.

l4 m'med H. How

Feb. 1, 1949. w. H. HOWE 2,460,503

APPARATUS FOR MEASURING LIQUID LEVEL OR SPECIFIC GRAVITY Filed Feb. 26, 1945 3 Sheets-Sheet 2 INVENTOR.

Wilfred fiflowe 96 BY 6%,MgZ Q T NEYS Feb. '1, 1949. w. H. HOWE APPARATUS FOR MEASURING LIQUID LEVEL OR SPECIFIC GRAVITY 3 Sheets-Sheet 3 Filed Feb. 26, 1945 may RNEYS INVENTOR. Wl'f'rea ff. Howe Q q. v

flag 6341 4] 4 WLI a Patented Feb. 1, 1949 APPARATUS FOR- MEASURING LIQUID LEVEL OR SPECIFIC GRAVITY Wilfred H. Howe, Sharon, Masa, assignor to The Foxboro Company, Foxboro, Masa, a corporation of Massachusetts Application February 26, 1945, Serial No. 579,721

.6 Claims.

This invention relates to apparatus for. measuring and indicating, recording or controlling the level or specific gravity of a liquid, liquid mixture or suspension of solids in a liquid, all of which are herein referred to as liquids. More particularly, the invention relates to apparatus for measuring the buoyancy of a float that is at least partially immersed in a liquid, as a measure oi the specific gravity or level of the liquid.

The level of a liquid is commonly measured by use of the well-known gauge glass wherein the liquid level is determined by a direct observation. There are, however, a number of situations in which it is inconvenient or impossible to use the conventional gauge glass. Thus there are cases 1 where it is desired to have an indication of liquid level in a tank or other vessel at a point remote from the tank and perhaps a considerable distance above or below the tank. It is apparent that in such cases a gauge glass cannot well be used.

In one conventional method oi determining the specific gravity of a liquid a float oi predetermined mass and volume is submerged in the liquid whose specific gravity is to be determined,

and the apparent weight 01! the submerged float is measured as a measure of the specific gravity of the liquid. The apparent weight of the float will differ from its known actual weight by an amount equal to the weight of the liquid (115- strain sensitive elements for measuring the apparent weight of a float submerged in a liquid and thereby determining the specific gravity of the liquid.

It is another object of the invention to provide apparatus for measuring the level 01 a liquid by means of a float floating on the surface of the liquid.

It is still another object of the present invention to provide improved apparatus for measuring and indicating, recording or controlling the level or specific gravityoi a liquid, which apparatus is highly resistant to corrosion and is not undesirably ailected by variation in static pressure.

It is a further object or the invention to pro- 2 vide apparatus for measuring the level or specific gravity or a liquid or such a character that an accurate indication of the liquid level or specific gravity may be obtained at a point remote from 5 the point of measurement.

It is still another object of the invention to provide apparatus for accurately measuring the liquid level or specific gravity of a liquid under high pressure.

Other objects of the invention will be in part obvious and in part pointed out hereafter.

The apparatus shown in the drawings comprises, in general, a float which. in cases where the liquid level is to be measured, is so selected as to weight and volume that it will float approximately half submerged in the liquid of which the level is to be measured. In cases where the specific gravity of the liquid is to be measured, a float is completely submerged in the liquid. In either case the float is rigidly connected to a flexure member in such manner that as the level or specific gravity of the liquid changes, the resulting change in buoyancy of the float creates a force that is applied to the fiexure member to bend it. This variation in the buoyancy oi the float is measured by measuring the strain it produces in the fiexure member.

Apparatus incorporating a preferred embodiment of the present invention as applied to the problem oi! measuring liquid level is illustratedin the accompanying drawings. It will be apparent to those skilled in the art from the foregoing statements that essentially the same apparatus can be used for measurement of specific gravity provided that the float is maintained completely submerged in the liquid, or if the float is only partially immersed, that the liquid level be maintained constant.

Figure 1 is a sectional elevational view showin a float and flexure member having an electrical strain gage element mounted thereon;

Figure 2 is an enlarged vertical section of the flexure member taken along the lines 2-4 of Figure 1; a

Figure 3 is an enlarged top plan view of a-portion or the axial core oi! the flexure member showing certain details or the electrical strain gage element;

Figure 4 is an electrical circuit that may be used to measure variations in the resistance of the strain gage element;

Figure 5 is a top plan view partly in section of another form of flexure member;

Figure 6 is a vertical section taken on the aasqsos 3 line 8-. cl Figure 5 and showing further details of the fiexure member of Figure 5;

Figure 7 is an enlarged vertical section taken on the line 1'l of Figure 6.

Referring to the drawings and particularlyto Figure 1, numeral l0 designates a tank or other vessel containing a body of liquid of which the level is to be measured. The tank Iii is connected by pipes l2 and It to a conventional float chamber it in such manner that when the level in the tank is at the desired point, liquid rises to approximately the midpoint of the float chamber it as indicated at IS. The chamber i6 at-a point above the liquid level I8 is provided with a flanged connection 20 to which is bolted a frustoconical collar 22. The opening at the right-hand end of collar 22 is closed by a plate 24 which is bolted to the collar 22 and has mounted in fixed relation thereto one end of a fiexure member or cantilever 26.

At its other end the flexui'e member 28 is connected to a float 2e, preferably selected to float half submerged in the body of liquid in the chamber it when the liquid half fills the chamber. That is, when the liquid inthe chamber is at mid-position, the float exerts neither an upward nor a downward force on the fiexure member 28. The fioat 28 is provided with a depending extension 36 that passes through a perforated plate 32 fixed to the wall of chamber 58. The p pose of the plate 32 is to maintain the float properly centered within the float chamber when the fioat is buoyed up by the rising level; and to act as a bane to prevent or minimize the effect of surges of liquid on the float. A guide plate at fixed to the casing of float-chamber iii surrounds the upper portion of float 22 to provide additional support and guidance for the float. The arrangement is such that variation of the liquid. level varies the buoyant force it exerts on the float held by the fiexure member, and

sistance wire I2 arranged in an elongated sinuous pattern on a piece of thin paper '4 and bonded to the paper with an adhesive such as a glyptal resin in such manner that each segment of the wire throughout its effective length is firmly bonded to the paper. It is desirable that a double filament be used to minimize or eliminate undesirable inductive effects it alternating current is used to measure the resistances oi. the elements.

The paper at having the winding 01' resistance wire thereon is then firmly bonded to the top of fiexure is with the lengths of resistance wire parallel to the axis of the fiexure member 26 so that as the fiexure 58 is strained, the resistance wire as is proportionally strained and its resistance thereby varied.

The element 58 is constructed similarly to the element as and is similarly bonded to the bottom of ilexure 36. It is apparent that with the construction described above iiorce exterted by the float 28 in response to changes in liquid level will strain the-fiexure member as. Because of the configuration ofthe fiexure 5d, a major portion of the strain of the member 26 will occur in the fiexure. If the liquid level moves downwardly, float as may be caused to exert a downward force on the end of fiexure member 2t, and element 38 will be put under tension and its electrical re sistance thereby increased; whereas the element at will be put under compression and its elsetrical resistance thereby decreased. If, on the other hand, the liquid level moves upwardly, float 28 may be caused to exert an upward force and the element 48 will be compressed thereby decreasing its resistance, whereas element tit will be stretched and its resistance increased. The buoyancy of the float 28 is desirably selected in such manner that when the liquid level it) is at the float in turn flexes the fiexure member 28.

The flexure member 22 comprises a rod 36 connected at one end to the top of float 28 and at its other end threaded into the rigid core at. Reierrlng now additionally to Figures 2 and 3, the core as comprises cylindrical and portions (it and $2 and a neck id extending therebetween. The central portion of the neck 54 is cut away at the top and bottom to provide a less rigid section 415 of approximately rectangular cross section commonly called a flexure. The core 36 may be protected from the atmosphere of the liquid by a flexible cover tube 38, preferably made of a relatively flexible material such as rubber, suitable plastic, or the like. or alternatively may be a metal bellows. Additional protection may be provided at the left-hand end of the tube 88 by a flexible diaphragm $8 of similar material. The diaphragm is clamed between the flanged connection 2t and collar 22 in such manner as to seal 01! the space between tube 28 and collar 22 and prevent liquid or vapor from the chamber it from entering this space.

Mounted on the top and bottom oi? the fiexure 48 are a pair of electrical strain gage elements 48 and 50 that may be generally of the type shown in the United States Patent 2,292,549 to Edward E. Simmons, Jr. The elements 48 and 50 preferably comprise a winding of fine electrical resistance wire suitably bonded to a thin supporting sheet which is in turn bonded to the flexure 46. The wire used is preferably of the order of a mil in diameter. As shown in Figures 2 and 3, the element ll comprises a double filament of rethe desired point the fiexui'e member 26 is in un= stressed condition. The change in resistance of the elements 58 and tit as they are strained then becomes a measure of the departure of the liquid level from a desired predetermined value and the resistances of the elements (it and Eli may be measured to determine variations in the liquid level. since the elements 418 and 5d are bonded to the fiexure (it throughout their efiective length, the elements will be uniformly strained as the fiexure 38 bends.

Referring now to figures 1 and d, the elements .8 and as are connected to a resistance measuring device 55. Various resistance measuring devices well-known in the art may be used, but for pur= poses of illustration a simple measuring circuit is shown in Figure 4. As shown in'Figure s the elements 62 and be are placed in adjacent arms of a Wheatstone bridge, the other two arms of the bridge containing a fined resistance 56 and a variable resistance iii A galvanometer 6b is connected across the bridge between the common lead of the elements (it and 55B, and the common connection of resistances 5t} and lid. Electrical energy from a suitable source such as the battery 82 is supplied to the other two corners M and are efiectively in opposed relation and hence the effects of their resistance changes are additive.

As the elements 48 and 5i! are strained and their resistances thereby varied the bridge is unbalanced and the needle 01' galvanometer 88 defiects. The variable resistance 58 is then adjusted to bring the galvanometer needle back to its zero or null position.

able resistance 58 and is provided with an index I8 that cooperates with a stationary reference scale I8. When the bridge becomes unbalanced, knob 14 may be rotated to bring needle 88 back to the null position I2. Since the elements 48 and 88 are connected in opposition to one another, the setting of resistance 58 is proportional to the arithmetic sum of the changes in resistance of the elements and the extent to which the knob I4 must be rotated to rebalance the bridge is a measure of the departure of the liquid level from its initial position. .The reference scale I8 may be calibrated directly in termsof liquid level, in which case the index I8 indicates on scale I8 the level of the liquid in tank I8.

It is to be understood that the measuring circuit shown in Figure 4 described above is included for purposes of illustration only. The variation in resistance of elements 48 and 58 is in many cases not very great and hence to obtain the required accuracy of measurement it may be desirable in some cases to use more elaborate measuring devices that are capable of giving considerably more accuracy than the device described above. One type of measuring apparatus that may be used with particular advantage in conjunction with apparatus embodying the present invention is disclosed in a copending application of Wilfred H. Howe and Robert W. Cushman, Serial No. 496,438, filed July 28, 1943.

It may be noted that variations in the resistance of the strain responsive elements 48 and 58 may be used to control the liquid level in tank I8. Thus the elements 48 and 58 may be connected to a potentiometer controller of any suitable and well-known type or an electronic instrument such as that shown in the above-identified Howe et 8.1. application to adjust a valve in a pipethrough which flows a fluid affecting the level of liquid in tank I8.

Referring now to Figures 5, 6, and 7, showing an alternative construction, the float 28 is caused to vary the strain on the strain sensitive elements bymoving up and down wih the rise and fall of the liquid level. The float is not rigidly supported, but is movably supported by the rod 84 which is in turn fixed to end block 82. This block is supported in the left end of a flexible member or tube 88, which is similar to tube 88, and which is firmly fixed 'at its right-hand end in plate 24. The ends of tube 88 are closed by the blocks 82 and 84 to form an enclosed space within this tube containing a modified form of flexure member,-i. e., the electrical strain gage elements themselves comprise the flexure member. The

- elements, comprising the resistance wires 88 and 88, are arranged in the form of a V as best shown in Figure 6. At their right-hand end the wires 88 and 88 are connected tothe adjustable blocks 88 and 82 which are mounted on and internally threaded to receive the threaded shaft 84. The shaft 94 is provided with right-handed and lefthanded threadssothat as the shaft is rotated the blocks 88 and 92 move towards each other or away from each other depending upon the direction of rotation of the shaft. At the lower end of shaft 84 there is a knob 88 by means of which the distance between the blocks 88 and 82, and hence the distance between the right-hand ends of wires 88 and 88 may be adjusted. Shaft 84 is 6 mounted in aframe 88 which is fixed to the block 84.

The junction of the wires 88 and 88 at the apex of the V is connected to a double U-shaped member I88 pivotally supported in the 'frame 88 in such manner that as rod 34 is moved by the float 28 in response to changes in liquid level, the junction I82 moves upwardly or downwardly to elonsate one of the wires 88 and 88 and to release a part of the tension in the other wire to permit it to contract. The junction I82 of the wires 88 and 88 is connected to the pivoted member I88 through a light spring I84 and an adjusting screw I88 which permits adjustment of the tension on the wires. Secured to the pivoted U-shaped member. I88 there is a rod I88v having'a spherical end II8 located between the arms of a U-shaped member I I2 which is fixed to the block 82.

The midsection of tube 88 may be of a metal bellows construction to provide such flexibility that practically all the motion imparted to the rod 34 by the float 28 is transmitted to the pivoted member I88.

The operation of the fiexure means of Figures 5 and 7 is apparent from the above description. If the liquid level in float chamber I8 rises, it carries the float upward, thus moving U-shaped member H2, sphere H8 and pivoted member I88 upwardly. Junction I82 will move upwardly to increase the tension in the wire 88, thus increasing its resistance, and to decrease the tension in wire 88, thus reducing its resistance. When the liquid level drops, the resistance of wire 88 will increase correspondingly and the resistance of wire 88 will decrease correspondingly. The junction I82 is connected by a wire I I4 to the binding post H8 and the right-hand ends of resistance elements 88 and 88 are connected by the wires H8 and I28 to the binding posts I22 and I24 respectively. The binding posts II8, I22, and I24 are connected respectively to the leads I28, I28, and I88 which pass through the conduit I32 in block 84. Variations in the resistances of arms 88 and 88 may be measured and an indication of liquid level obtained in the manner described in connection with Figures 1 through 4, i. e., by connecting the resistances 88 and 88 in the arms of a suitable Wheatstone bridge circuit.

To protect the flexure member 28 (Figure 1) and the flexible tube 88 and its associated parts surfaces thereof, whereupon the sto prevents further movement of the rod 34 and float 28. Referring to Figure 1, the length of the slot is such as to prevent any force exerted by the float that would excessively strain the flexure member 28; whereas in the Figure 5 construction, the length of the slot is such as to prevent any movement of the float that would damage the tube 88 and its associatedparts.

, 3 From the above description it is apparent that the apparatus of the present invention is par ticularly well adapted for measuring liquid level in those cases where it is inconvenient or impossible to use the conventional type of gauge glass. Also essentially the same apparatus may be tlon of liquid level or specific gravity may be obtained at a remote point and the liquid level or specific gravity of a liquid under high pressure may be measured with advantage since the electrical measuring element may be located inside the pressure chamber.

Since many embodiments might be made of the present invention and since many changes might be made in the embodiment described above, it is to be understood that the foregoing description is to be interpreted as illustrative onlyand not in a limiting sense.

I claim:

1. In apparatus for measuring the buoyancy of a float at least partially immersed in aliquid as a measure of a characteristic of said liquid such as specific gravity and liquid level, in combination, a flexure member, one end or which is rigidly mounted in a fixed support and the other end of which is connected to said float whereby variations in the force exerted by said float in response to variations in the value of said characteristic flex said flexure member, a first resistance element fixed to the top of said flexure member, a second resistance element fixed to the bottom of said flexure member, said resistance elements being so fixed to said member that they are strained as said member is flexed and their electrical resistances are thereby varied, means responsive to the arithmetic sum of the changes in the electrical resistance of said elements for indicating the value of said characteristic of said liquid, and flexure means for sealing said elements from the atmosphere of the liquid.

2. In apparatus for measuring the buoyancy of a float at least partially immersed in a liquid as a measure of a characteristic of said liquid such as specific gravity and liquid level, in combination, a flexure member having one end fixed to a rigid support and connected at its other end to said float whereby variations in the force exerted specific gravity.

8 4. In apparatus for measuring the buoyancy of a float at least partially immersed in a liquid as a measure of a characteristic of said liquid by said float in response to variations in the value of said characteristic flex said flexure member, a resistance element connected to spaced points on saidflexure member to cause said element to be strained and its electrical resistance thereby varied as said flexure member is flexed, a closed flexible protecting tube surrounding said flexure member and said resistance element to protect said flexure member and resistance element from said liquid and its vapor, and means responsive to the electrical resistance of said element for indicating the value of said characteristic of said liquid. a

3. In apparatus for measuring the buoyancy of a float at least partially immersed in a liquid as a measure of a characteristic of said liquid such as specific gravity and liquid level, in combination, a flexure member rigidly supported at one end and connected near its other end to said float to cause the buoyancy of said float to flex said fiexure member, an electrical resistance element fixed to said flexure member throughout the length of said resistance element to cause said element to be uniformly strained throughout its length and its electrical resistance thereby varied as said flexure member is flexed, and means responsive to changes in the electrical resistance of said element for indicating the value of said characteristlc of said liquid.

such as specific gravity and liquid level, in combination, a cantilever connected near its free end to said float to cause the buoyancy of said float to flex said cantilever, a first electrical resistance element fixed to the top of said cantilever, a second electrical resistance element fixed to the bottom of said cantilever, said resistance elements being so fixed to said cantilever that they are strained as said cantilever is strained and their resistances are thereby varied, and means responsive to changes in the electrical resistances of said resistance elements for indicating the value of said characteristic of said liquid.

5. In apparatus for measuring the buoyancy of a float at least partially immersed in a liquid as a measure or a characteristic 1* said liquid such as specific gravity and liquid level, in combination, a cantilever connected near its free end to said float to cause the buoyancy of said fioat to flex said cantilever, a first electrical resistance element fixed to the top of said cantilever, a second electrical resistanceelement fixed to the bottom of said cantilever, said resistance elements being so fixed to said cantilever that they are strained as said cantilever is strained and their resistances are thereby varied, and means responsive to the arithmetic sum of the changes in the electrical resistances of said resistance elements for indicating the value of said characteristics of said liquid.

6. In apparatus for measuring the buoyancy of a float at least partially immersed in a liquid as a measure of a characteristic of said limud such as specific gravity and liquid level, in combination, a cantilever connected near its free end to said float to cause the buoyancy or said float to strain said cantilever, said cantilever including a flexure section of reduced cross-sectional area in which most or the strain of said cantilever is concentrated as said cantilever is flexed, an electrical resistance element connected to spaced points on said flexure section to cause said resistance element to be strained and its resistance thereby varied as said cantilever is strained, and means responsive to changes in the electrical resistance of said element for indicating the Value of said characteristic of said liquid.

WILFRED H. HOWE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Great Britain 1905 

